Ferrari are the only top four team yet to win this season, and both Charles Leclerc and Lewis Hamilton, a seven-times world champion, have openly expressed their frustration.
Hamilton has not stood on the podium since he joined from Mercedes in January and has called the season the worst of his career.
On Friday, the Briton qualified 18th for the sprint race, after lining up last in Las Vegas the previous weekend.
"Quite early in the season, McLaren was so dominating in the first four or five events that we realised it would be very difficult for 2025," Vasseur told reporters before sprint qualifying at the Qatar Grand Prix.
"It meant that we decided very early in the season, I think it was the end of April, to switch (the development focus) to '26. It was a tough call.
"Perhaps I also underestimated a little bit the call on the psychological side, because when you still have 20 races to go, or 18 races to go, and you know that you won’t bring any aero development, it's quite tough to manage psychologically."
Ferrari still brought some mechanical upgrades and tried to tighten up operationally, but have been concentrating their efforts on the car for next year, the start of a new engine and rules era.
"I’m still confident with the call that we made," said Vasseur, whose team last won a championship in 2008 and finished runners-up last year.
Vasseur said he perfectly understood his drivers' frustration and did not blame them.
"We have exactly the same frustration in the garage or on the pit wall. The most important is to react after the frustration," added the Frenchman.
"I think, honestly, the frustration sometimes is good... you can’t imagine, and I would be even more frustrated, if the drivers were happy when we have a tough weekend."